Browsing by Subject "genetic diversity"

Rapid environmental changes over the last 100 years have led to substantial range shifts across taxonomic groups. Understanding what facilitates successful shifts is important for predicting ecological consequences and planning efficient conservation actions. Interestingly, the very process of range expansion can affect the success of the shift by causing genetic changes in the expanding populations. Theory predicts that without sufficient gene flow, repeated founder events and strong genetic drift can result in allele frequency gradients and loss of genetic diversity along the expansion axis. Empirical studies testing these expectations in environment-driven range shifts are still relatively scarce, and how range expansions affect genetics in highly mobile species remains unclear. In this study, I investigated the genetic consequences of a recent range expansion in a long-distance migratory passerine, the reed warbler (Acrocephalus scirpaceus). Utilizing genome-wide data from restriction site-associated DNA sequencing (RAD-seq), I studied whether the expansion was reflected in either population structure or genetic diversity of the recently established Finnish range edge population. Despite philopatry and genetic differentiation to the range core populations, principal component analysis (PCA) and a model-based Bayesian clustering approach (fineRADstructure) revealed a lack of spatial population structure along a putative colonization route. Levels of genetic diversity, based on expected heterozygosity, nucleotide diversity, and private allele count, were found to be very similar between range edge (Finland) and range core (Central Europe). The results likely indicate high levels of gene flow both within the new population and across greater spatial distances during or after the range expansion. Due to a detected sequencing batch effect, however, the exact diversity estimates must be considered preliminary. These findings suggest that species with high enough dispersal propensity may escape the predicted genetic costs of range expansions, retaining high levels of genetic variation at range margins. This study provides valuable insights for understanding range shifts in mobile taxa, and highlights the need to investigate further the traits of species that enable the preservation of evolutionary potential during range shifts.

The three-spined stickleback (Gasterosteus aculeatus) is a model organism for studies of parallel evolution in the wild; marine stickleback populations have repeatedly colonized and successfully adapted to different brackish and freshwater habitats. During Pleistocene glaciations, three-spined stickleback populations inhabiting high-latitude areas of Europe were eradicated, whereas populations residing in (or moving to) the south persisted in refugia. After the retreat of the ice sheets covering northern Europe, the high-latitude areas became recolonized by migration from south, and hence, today’s northern European populations are relatively young. Population genetic studies of European three-spined sticklebacks have usually been conducted at high-latitude areas where freshwater populations are typically less than 10.000 years old. Few studies have focused on southern populations, where more of the ancestral diversity is likely to reside. These studies have utilized a limited number of microsatellite markers and mtDNA sequence fragments, whereas studies of southern populations focusing on genome-wide diversity, in particular from the edge of the southern distribution limit in the Iberian Peninsula, are still missing. Here, I wanted to cover this gap in knowledge by carrying out an empirical and statistical study with RAD-seq data from southern and northern European populations of three-spined stickleback. The main aims of this study were two-fold. First, to investigate whether the southern European freshwater populations of the three-spined stickleback – which currently lack or have limited connection to ancestral marine populations carrying most of the standing genetic variation (SGV) – have lost genetic diversity due to population bottlenecks and inbreeding as compared to their northern European counterparts. Second, to compare the degree of genetic parallelism in southern vs. northern European populations in genomic regions which have been shown to be consistently associated with freshwater colonization in earlier studies. Under the assumption that the lack of continued access to SGV in the ancestral marine population reduces the likelihood of parallel evolution, I hypothesized that the degree of genetic parallelism in genomic regions subject to positive selection in freshwater environments is lower in the southern than in northern European populations. However, if a reduction in genetic diversity and/or cessation of gene flow between southern European freshwater and marine populations occurred following freshwater adaptation, the opposite pattern could be expected. I paid particular attention to chromosomal inversions associated with marine-freshwater adaptations identified in previous studies. The results confirmed my expectation of reduced genetic diversity in southern as compared to northern European stickleback populations. On the other hand, and contrary to what I expected, analysis of clusters of global parallelism involved in freshwater adaptation revealed that southern European populations exhibit a higher degree of genetic parallelism in response to freshwater colonisation than those from northern Europe. This suggests that the loss of genetic diversity in southern populations has occurred after they had adapted to freshwater environments, explaining the high degree of genetic parallelism in spite of the current low levels of genetic diversity. In addition, it could be that selection pressures in south are more homogenous than in north, which would also explain the higher degree of genetic parallelism observed in southern Europe. The findings presented here, challenge the current paradigm that parallel evolution is unlikely in populations with low genetic diversity and that have experienced recent bottlenecks.